1. Field
Example embodiments relate to methods and apparatuses for operating nuclear reactors and for determining power levels in the nuclear reactors. Also, example embodiments relate to methods and apparatuses for operating nuclear reactors and for determining power levels in the nuclear reactors that may include two or more electrical conductors, one or more signal devices, and/or an analyzer.
2. Description of Related Art
FIG. 1 is a sectional view, with parts cut away, of reactor pressure vessel (“RPV”) 100 in a related art boiling water reactor (“BWR”). During operation of the BWR, coolant water circulating inside RPV 100 is heated by nuclear fission produced in core 102. Feedwater is admitted into RPV 100 via feedwater inlet 104 and feedwater sparger 106 (a ring-shaped pipe that includes apertures for circumferentially distributing the feedwater inside RPV 100). The feedwater from feedwater sparger 106 flows down through downcomer annulus 108 (an annular region between RPV 100 and core shroud 110).
Core shroud 110 is a stainless steel cylinder that surrounds core 102. Core 102 includes a multiplicity of fuel bundle assemblies 112 (two 2×2 arrays, for example, are shown in FIG. 1). Each array of fuel bundle assemblies 112 is supported at or near its top by top guide 114 and at or near its bottom by core plate 116. Top guide 114 provides lateral support for the top of fuel bundle assemblies 112 and maintains correct fuel-channel spacing to permit control rod insertion.
The coolant water flows downward through downcomer annulus 108 and into core lower plenum 118. The coolant water in core lower plenum 118 in turn flows up through core 102. The coolant water enters fuel assemblies 112, wherein a boiling boundary layer is established. A mixture of water and steam exits core 102 and enters core upper plenum 120 under shroud head 122. Core upper plenum 120 provides standoff between the steam-water mixture exiting core 102 and entering standpipes 124. Standpipes 124 are disposed atop shroud head 122 and in fluid communication with core upper plenum 120.
The steam-water mixture flows through standpipes 124 and enters steam separators 126 (which may be, for example, of the axial-flow, centrifugal type). Steam separators 126 substantially separate the steam-water mixture into liquid water and steam. The separated liquid water mixes with feedwater in mixing plenum 128. This mixture then returns to core 102 via downcomer annulus 108. The separated steam passes through steam dryers 130 and enters steam dome 132. The dried steam is withdrawn from RPV 100 via steam outlet 134 for use in turbines and other equipment (not shown).
The BWR also includes a coolant recirculation system that provides the forced convection flow through core 102 necessary to attain the required power density. A portion of the water is sucked from the lower end of downcomer annulus 108 via recirculation water outlet 136 and forced by a centrifugal recirculation pump (not shown) into a plurality of jet pump assemblies 138 (only one of which is shown) via recirculation water inlets 140. Jet pump assemblies 138 are circumferentially distributed around core shroud 110 and provide the required reactor core flow.
As shown in FIG. 1, a related art jet pump assembly 138 includes a pair of inlet mixers 142. A related art BWR includes 16 to 24 inlet mixers 142. Each inlet mixer 142 has an elbow 144 welded to it that receives water from a recirculation pump (not shown) via inlet riser 146. An example inlet mixer 142 includes a set of five nozzles circumferentially distributed at equal angles about the axis of inlet mixer 142. Each nozzle is tapered radially inwardly at its outlet. Jet pump assembly 138 is energized by these convergent nozzles. Five secondary inlet openings are radially outside of the nozzle exits. Therefore, as jets of water exit the nozzles, water from downcomer annulus 108 is drawn into inlet mixer 142 via the secondary inlet openings, where it is mixed with coolant water from the recirculation pump. The coolant water then flows into jet pump assembly 138.
FIG. 2 is a top plan view of a related art core 200. Core 200 may include fuel bundles 202, peripheral fuel bundles 204, and/or control rods 206. Two or more of fuel bundles 202 may be included in fuel bundle assemblies 208. Core 200 may include, for example, hundreds or thousands of fuel bundles 202 and/or tens or hundreds of peripheral fuel bundles 204. As shown in FIG. 2, for example, core 200 may include approximately one thousand and twenty-eight (1,028) fuel bundles 202, approximately one hundred and four (104) peripheral fuel bundles 204, and/or approximately two hundred and sixty-nine (269) control rods 206.
The distribution of fuel bundles 202, peripheral fuel bundles 204, and/or control rods 206 in core 200 may or may not be symmetric. Additionally, if symmetry exists, it may include one or more of mirror-image symmetry, diagonal symmetry, rotational symmetry, translational symmetry, quadrant symmetry, and octant symmetry. As shown in FIG. 2, for example, one or more control rods 206 may be disposed in or near a geometric center of core 200.
Core 200 also may include one or more types of neutron monitors. These monitors may include, for example, one or more source range monitors, one or more intermediate range monitors, and/or one or more power range monitors. In a related art BWR, the one or more source range monitors may be fixed or movable. Similarly, in a related art BWR, the one or more intermediate range monitors may be fixed or movable.
At least some of the overall range of a related art source range monitor and/or a related art intermediate range monitor may be covered by a startup range neutron monitor (“SRNM”) or wide range neutron monitor (“WRNM”). Similarly, at least some of the overall range of a related art intermediate range monitor and/or a related art power range monitor may be covered by a local power range monitor (“LPRM”). In a related art BWR, the SRNMs and/or the LPRMs may be fixed.
Core 200 may include, for example, tens of SRNM detectors and/or tens or hundreds of LPRM detectors. Although not shown in FIG. 2, core 200 may include, for example, approximately twelve (12) SRNM detectors. As shown in FIG. 2, for example, core 200 may include approximately two hundred and fifty-six (256) LPRM detectors in approximately sixty-four (64) LPRM assemblies 210. For example, one or more LPRM assemblies 210 may include four LPRM detectors (i.e., each LPRM assembly 210 may include four LPRM detectors).
FIG. 3 is a side elevation view of a related art LPRM assembly 300. As shown in FIG. 3, guide tube 302 of LPRM assembly 300 may penetrate core plate 304, allowing LPRM assembly 300 access into core 306. One or more guide rings 308 of LPRM cover tube 310 may guide the insertion of LPRM assembly 300 into guide tube 302. Guide tube 302 may be sealed by gland seal 312 and/or flange 314.
LPRM assembly 300 may include, for example, four LPRM detectors (not shown) and/or six connectors. First connector 316 may connect to a first LPRM detector, second connector 318 may connect to a second LPRM detector, third connector 320 may connect to a third LPRM detector, and/or fourth connector 322 may connect to a fourth LPRM detector. One or more of first connector 316, second connector 318, third connector 320, and fourth connector 322 may be a connector manufactured by the LEMO company, based in Switzerland, and known as a LEMO plug or LEMO receptacle. For example, one or more of first connector 316, second connector 318, third connector 320, and fourth connector 322 may be a size 1 LEMO receptacle.
Fifth connector 324 may connect to a gamma thermometer (not shown) of LPRM assembly 300. Fifth connector 324 may be, for example, a size 3 LEMO plug. Sixth connector 326 may be, for example, a calibration tube associated with a traversing in-core probe (“TIP”) (not shown).
A TIP is a gamma- or neutron-sensitive device that may be fully inserted into a nuclear reactor core, then withdrawn in a measured manner to determine the gamma or neutron flux at axial elevations in the core. TIP readings are continuous, but typically are digitized at set intervals (e.g., 1″) and then combined into one value representative of the power in a node (e.g., a 6″ segment). Since in-core structures such as spacers may affect the local gamma or neutron flux, dips in power may be correlated with known spacer locations to enhance proper alignment of the data. At the beginning of a TIP set, all TIPS (typically from 3 to 5) may be run through a common core radial location to allow the different TIPs to be normalized to each other. Data collected from each radial location around the core may then be normalized. This is commonly called “core adaption” and is generally no longer used. Instead, core physics computer programs may calculate the core radial power distribution, and the TIP readings may be used to allocate the power axially in each location. This process is commonly called “shape adaption.” For this process, it may be necessary that each TIP read consistently from top to bottom in each location, but not necessarily consistently from one radial location to another. In other words, inter-calibration of instruments may not be required for shape adaption.
In related art LPRM assembly 300, the first, second, third, and fourth LPRM detectors may be disposed in a substantially vertical arrangement. The substantially vertical arrangement may include spacing between the first, second, third, and fourth LPRM detectors. The spacing may be of the same size, or two or more different sizes. The substantially vertical arrangement may be, for example, approximately the same in each LPRM assembly 300. The substantially vertical arrangement may allow the first, second, third, and fourth LPRM detectors to monitor neutron flux (typically thermal neutron flux) at four different heights (or locations) in core 306. The four different heights (or locations) may be, for example, approximately the same in each LPRM assembly 300.
As is known by a person having ordinary skill in the art (“PHOSITA”), LPRM detectors typically include a cathode having fissionable material coated on the cathode. The fissionable material may be a mixture of U234 and U235. The U235 is used to provide a signal proportional to the thermal neutron flux. But due to the extremely high thermal neutron flux in the nuclear reactor core, the U235 is subject to burnout, which may cause the LPRM detector reading corresponding to a constant thermal neutron flux to gradually decrease over time. The U234 may absorb thermal neutrons to become U235, lengthening the life of the LPRM detector. Eventually, however, the LPRM detector reading corresponding to a constant thermal neutron flux will still gradually decrease over time.
A gamma thermometer may provide a capability to calibrate an associated LPRM detector. During steady-state operation, gamma flux typically is proportional to thermal neutron flux. Thus, a gamma thermometer—located near the associated LPRM detector—can measure local gamma flux during a steady-state heat balance, as known to a PHOSITA. The local gamma flux can be related to the proportional thermal neutron flux and the associated LPRM detector can be calibrated based on the related proportional thermal neutron flux. Currently, however, gamma thermometer technology is expensive and/or may provide a relatively limited number of temperature-compensation measurements over the height of core 306.
Although older technology than the gamma thermometer, a TIP can provide an alternate and/or supplemental vehicle to calibrate LPRM detectors. As known to a PHOSITA, a TIP essentially is a system that includes a mobile thermal neutron detector that may be temporarily positioned near an LPRM detector to be calibrated. During a steady-state heat balance, for example, the LPRM detector can be calibrated to the reading of the mobile thermal neutron detector. Because the TIP is mobile, it can be moved out of the extremely high thermal neutron flux in the nuclear reactor core. Thus, a TIP normally is not subject to the burnout problems of the LPRM detectors. However, TIPs are complex systems that are subject to mechanical and electrical failure, require frequent maintenance and repair, and raise numerous radiation exposure and contamination issues.
Various solutions to the problem of determining power levels in nuclear reactors have been proposed, as discussed, for example, in U.S. Pat. No. 4,614,635 (“the '635 patent”), U.S. Pat. No. 4,725,399 (“the '399 patent”), U.S. Pat. No. 4,915,508 (“the '508 patent”), and U.S. Pat. No. 5,015,434 (“the '434 patent”). The disclosures of the '635 patent and the '434 patent are incorporated in the present application by reference. However, these various solutions do not include methods and apparatuses for operating nuclear reactors and for determining power levels in the nuclear reactors, wherein the compensation of LPRM detectors may be performed simply, with reduced cost, and/or with a relatively large number of compensation measurements over the height of the core.